1. Field of the Invention
This invention relates generally to electronic circuits and, more particularly, to electronic circuits which provide common-mode feedback signals.
2. Description of the Related Art
For high performance signal conditioning systems, many analog circuits use dual output differential amplifiers. Differential amplifiers amplify the difference between inverting and non-inverting input signals and allow the rejection of noise from the substrate and clock signals coupled to the amplifier. However, differential amplifiers also amplify the average of the inverting and non-inverting input signals which is generally referred to as the common-mode.
The common-mode is characterized by the common-mode rejection ratio (CMRR) which is defined as the ratio of the differential to the common-mode gain. Hence, the CMRR can be increased by reducing the common-mode gain. In some differential amplifiers, the common-mode gain can be reduced by using switched capacitor circuitry, as disclosed in U.S. Pat. No. 6,400,301. The common-mode gain can also be reduced by using continuous time circuitry, as disclosed in D. A. Johns and K. Martin, Analog Integrated Circuit Design, John Wiley and Sons, New York, 1997, Pgs 287–291.
In a typical differential amplifier circuit, the the amplifier and its common-mode feedback (CMFB) circuitry are separate from any circuitry that may follow the differential amplifier. For example, in some analog-to-digital converter (ADC) applications the differential amplifier drives a comparator that compares a sample of the analog input signal to a reference voltage, which is typically a positive or negative differential voltage.
One type of comparator is known as a quad or differential difference comparator. These comparators receive input signals (VS+ and VS−) and reference signals (VR+ and VR−). Comparators of this type have an offset that is dependent on the difference between the common-mode of the input and reference signals. As the magnitude of the offset increases, the comparator's gain decreases, which reduces its effectiveness to operate as a comparator.
To increase the CMRR of the quad comparator, external CMFB circuitry is typically added between the differential amplifier and the comparator to establish the differential amplifier's common-mode output voltage. The CMFB circuitry is designed to make the common-mode voltages of the amplifier's output signal and comparator's reference signal the same. The external circuitry, however, increases the physical size of the circuit and makes the overall design more complicated.